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The filmmaker, who co-directed “Tanner Hall” — the 2009 drama starring Rooney Mara and Brie Larson — with Tatiana von Furstenberg, makes her solo debut with “Emanuel and the Truth about Fishes,” which premieres tonight at the Library Centre at Sundance.

Gregorini wrote the screenplay with “Tanner Hall” star Mara firmly attached; so involved with the development of the character and story that Gregorini says main character Emanuel has noticeable traces of Mara’s eclectic personality.

But the role called for a young teen, and as many indies do, the film took longer than expected to come together. Mara went on to make “The Girl with the Dragon Tattoo,” and had aged out of the part by the time “Emanuel” got under way, leaving Gregorini to search for a new lead.

“I looked for someone in L.A. for a long time — I looked at every girl available from 17 to 24,” Gregorini says. “I literally got on a plane, went to London and the minute (Scodelario) sat down next to me, I knew it was her. I knew she was my Emanuel.”

Primarily a television thesp (most notably in the U.K. show “Skins”) the 20-year-old British actress has been bandied about as a potential Sundance darling for weeks for her turn as a young girl who encounters a neighbor (Jessica Biel) bearing a striking resemblance to her dead mother. As she begins to get involved in the woman’s life, things get … weird.

And as for Biel, Gregorini isn’t the only one who thinks “Emanuel” could be a career-changer.

“I think she’s going to be a revelation to everyone,” Gregorini says, describing her character as “unhinged.” “(Jessica) saw the script and said she really wanted it. We met, and she offered to audition — someone at her level doesn’t have to do that — and she just blew me away. I was floored. It’s going to open people’s eyes to what a great dramatic actress she is.”

But bringing Scoledario and Biel into the project had diverted it from Gregorini’s original vision. In hindsight, she couldn’t be happier about that.

“The character of Emanuel took on all these other layers as Kaya took over the role,” says Gregorini, who recalls that at some point early on in the shooting of the film, she realized she was giving over to a collaborative process that was making her movie better.

“I’m quite a control freak,” she says with a laugh, “so that experience served me well.”